Pressure containers have in the past been largely constructed of a metal body and metal end closures. In the instance of the pressure container being an aerosol container, one end closure is contoured to receive and have crimped thereto a metal component referred to in the art as a mounting cup, which cup has affixed thereto a manually-actuable valve.
The metal body of the container is seamed along its length in the case of steel containers. This results, though avoidance is attempted, in an inner shape that is not truly cylindrical, the seam providing a discontinuity in the "true round" shape. In the case of aerosol aluminum containers, though seamless, the thin wall of the container is readily dented and a deviation from the "true round" results.
For many applications of an aerosol package system, for example, where a piston traversing the inner wall of the container body is a component of the package, a deviation from "true round" is undesirable. Where there is deviation from the "true round" a breakage in the seal between the inner wall of the container and the piston will occur with a concomitant loss or decrease in the efficiency of the discharge of the contents of the pressurized container.
Additional shortcomings of metal containers, often manufactured away from the site where the product is introduced into the container, is the shipment of the container to the filling site. Moreover, corrosion may be a problem necessitating a coating of the metal in order to make the inner surface of the container compatible with the product to be dispensed, and consequently and additional manufacturing operation.
The deficiencies of metal containers have resulted in an effort by marketers to replace the metal container with a plastic container.
Plastic pressure container have to date been manufactured by injection molding or blow molding processes. Both processes have serious drawbacks.
When injection molding a container, it is necessary that the body portion of the container have a draft or slope in order to eject the container from the mold. Further, and particularly with containers having a body portion with a length of conventional containers, such as beverage or aerosol containers, it is extremely difficult to fill the cavity defining the body portion of the container with the consequence that channeling or incomplete fill of the injection mold cavity results. As a consequence, in order to properly fill the cavity it is essential to use excessive temperature and pressure conditions, which result in a differential temperature profile over the length of the cavity and consequently stress and strain, warping and embrittlement of the molded container. Additionally, it is difficult to hold the core defining the inside wall of the body portion of the container properly centered with the result that the container wall is of varying thickness. Since permeation from within or external to the container is a function, among others, of the wall thickness, to compensate for a shift from true center of the cavity core, the injection mold cavity must be designed to provide a minimum wall thickness throughout. To assure the necessary minimum thickness necessarily results in a design of a wall thickness excessive to that necessary to properly contain the product.
Blow molding, necessarily, results in the wall of the pressure container being of uneven thickness since the pressure and temperature variations on the surface of the parison or preform is not uniform. Moreover, molecular weight variation in the parison and pre-form foreclose formation of a container having a substantially uniform wall thickness. Thus, as in an injection molding process, excessive amounts of plastic must be used in order to assure the minimum wall thickness necessary throughout the container to properly contain the product to be dispensed. Obviously, a variation in the wall thickness precludes formation of a body portion having an inner surface that is "true round" and consequently the container lacks usefulness as a container where the "true round" is essential to the dispensing of the product.
Further, in blow molding a container the end closures necessarily must be formed of the same plastic material. Further, in blow molding design, flexibility is limited. Moreover, in an aerosol-type container, where the top opening is smaller in diameter than the body portion of the container it is impossible to position a piston having a diameter substantially the same as the inside diameter of the container with the container.